An exhaust gas emitted from diesel engines contains PM (particulate matter) based on carbonaceous soot and SOF (soluble organic fraction) of high-boiling-point hydrocarbons. When such exhaust gas is released into the atmosphere, it may adversely affect human beings and the environment. For this reason, a PM-capturing ceramic honeycomb filter, which may be called “honeycomb filter” in short, has been disposed in an exhaust pipe connected to a diesel engine. One example of honeycomb filters for purifying an exhaust gas by removing particulate matter is shown in FIGS. 5 (a) and 5 (b). The honeycomb filter 10 comprises a ceramic honeycomb structure (simply called honeycomb structure) comprising porous cell walls 2 defining large numbers of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4, and an outer peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c alternately sealing the outlet-side-sealed flow paths 3 and the inlet-side-sealed flow paths 4 on the exhaust-gas-inlet-side end 8 and the exhaust-gas-outlet-side end 9 in a checkerboard pattern.
Ceramics for forming cell walls of this honeycomb structure are conventionally heat-resistant ceramics such as cordierite, silicon carbide, etc. Among them, cordierite is the optimum ceramic having good heat shock resistance, PM-capturing efficiency and small pressure loss, but it is disadvantageously subjected to local melting in the uncontrolled burning of the captured PM. The local melting undesirably results in the reduction of PM-capturing efficiency. Silicon carbide is resistant to melting erosion because of a higher melting point than that of cordierite, but it has a larger thermal expansion coefficient and poorer heat shock resistance than those of cordierite. Therefore, a honeycomb structure of silicon carbide should be formed by bonding pluralities of small-cross-section-area honeycomb structures, resulting in high production cost.
To solve such problems, attempts have recently been conducted to use aluminum-titanate-based ceramics having excellent heat resistance and low thermal expansion for honeycomb filters.
JP 2005-519834 A discloses a honeycomb filter made of about 50-90% by mass of aluminum titanate stabilized by Fe or Mg and about 10-50% by mass of strontium feldspar, and having a low thermal expansion coefficient, high heat shock resistance, high volume heat capacity, high communicating porosity, a large median pore diameter, and improved thermal stability at 800° C. or higher. It is described that this honeycomb filter can be produced by blending ceramic materials such as silica, alumina, strontium carbonate, titania, iron oxide, magnesium carbonate, etc. with organic components such as a plasticizer, a lubricant, a binder, a solvent, etc., molding the resultant mixture, and if necessary, drying and then sintering the resultant molding. It is also described that in this conventional technology, a titania source for synthesizing aluminum titanate is preferably rutile having a particle size of about 7-15 μm, and an Al2O3 source is preferably alumina having a particle size of about 10-25 μm.
WO 2005/018776 A discloses a sintered aluminum titanate honeycomb filter formed by sintering a starting material mixture comprising TiO2 (titania) and Al2O3 (alumina), and 1-10 parts by mass of alkali feldspar, a Mg-containing, spinel-type oxide, MgO or a Mg-containing compound convertible to MgO by sintering. It is described that this honeycomb filter is free from the problems of conventional aluminum titanate, which are thermal decomposition at 800-1280° C. and low mechanical strength, while retaining inherently high heat resistance and small thermal expansion coefficient. It is also described that in this conventional technology, the starting materials are preferably sufficiently mixed and pulverized as finely as possible to an average particle size of 30 μm or less, particularly 8-15 μm.
Both JP 2005-519834 A and WO 2005/018776 A disclose technologies of forming aluminum titanate free from the problems of thermal decomposition at 800-1280° C. and low mechanical strength while retaining inherently high heat resistance and low thermal expansion coefficient, by adjusting the types, compositions and amounts of additives added to TiO2 source powder and Al2O3 source powder.
However, when the technologies described in JP 2005-519834 A and WO2005/018776 A are used on large honeycomb structures of 100 mm or more in outer diameter and 150 mm or more in length, for instance, they suffer sintering cracking. When silica, strontium carbonate, iron oxide, magnesium carbonate, alkali feldspar, a Mg-containing, spinel-type oxide, MgO, or a Mg-containing compound convertible to MgO by sintering, etc. are added to the TiO2 source powder and the Al2O3 source powder, these additives form a liquid phase in the sintering process to cause densification, resulting in a large sintering shrinkage ratio. As a result, there is shrinkage difference in the honeycomb structure due to the temperature difference between a center portion and a peripheral portion, so that the honeycomb structure suffers sintering cracking. Such phenomenon is likely to occur more often as the honeycomb structure becomes larger. Sintering cracking occurs often particularly in honeycomb structures of 100 mm or more in outer diameter and 150 mm or more in length. Also, a larger sintering shrinkage ratio provides a smaller average pore size, failing to provide honeycomb filters with necessary porosity.